What "either-or" proposition did the Chauvelin present to Marguerite?

"What I want you to do to ensure Armand's safety is a very simple thing, dear lady."

"What is it?" Marguerite's hand seemed to convey to his, in response.

"To remain--on this spot, without uttering a sound, until I give you leave to speak. Ah! but I think you will obey," he added, with that funny dry chuckle of his as Marguerite's whole figure seemed to stiffen, in defiance of this order, "for let me tell you that if you scream, nay! if you utter one sound, or attempt to move from here, my men--there are thirty of them about--will seize St. Just, de Tournay, and their two friends, and shoot them here--by my orders--before your eyes."

Marguerite had listened to her implacable enemy's speech with ever-increasing terror. Numbed with physical pain, she yet had sufficient mental vitality in her to realize the full horror of this terrible "either--or" he was once more putting before her; "either--or" ten thousand times more appalling and horrible, that the one he had suggested to her that fatal night at the ball.

This time it meant that she should keep still, and allow the husband she worshipped to walk unconsciously to his death, or that she should, by trying to give him a word of warning, which perhaps might even be unavailing, actually give the signal for her own brother's death, and that of three other unsuspecting men.

She could not see Chauvelin, but she could almost feel those keen, pale eyes of his fixed maliciously upon her help less form, and his hurried, whispered words reached her ear, as the death-knell of her last faint, lingering hope.

"Nay, fair lady," he added urbanely, "you can have no interest in anyone save in St. Just, and all you need do for his safety is to remain where you are, and to keep silent. My men have strict orders to spare him in every way. As for that enigmatic Scarlet Pimpernel, what is he to you? Believe me, no warning from you could possibly save him. And now dear lady, let me remove this unpleasant coercion, which has been placed before your pretty mouth. You see I wish you to be perfectly free, in the choice which you are about to make."